An electric power supply is a device or system that is capable of providing electrical energy to a load—typically by converting electrical energy from one form to another to make the energy compatible with the load's requirements. For example, an electric power supply might convert 120 or 240 volt alternating current (“AC”) energy to lower-voltage, regulated direct current (“DC”) energy appropriate for use by an electronic device such as a computer system. Sometimes power supplies are integrated with the devices for which they supply energy. In other applications, power supplies are discrete components and can be internal or external to the load.
Switching electric power supplies (also known variously as switch-mode power supplies, switched-mode power supplies and other similar terms) are those that make use of active switching circuitry along with inductive elements to accomplish the energy conversion task with a minimum of energy loss. In switching power supplies that take their input from AC mains, a common configuration is to employ rectification circuitry and bulk capacitors to create a DC supply from the available AC input. This DC supply is then provided to one or more switching DC-DC output conversion circuits that generate desired DC output levels. Many well-known circuit topologies exist that achieve switching DC-DC conversion. Among them are boost converters, which accomplish DC-DC conversion with a resultant increase in the DC voltage level; buck converters, which accomplish DC-DC conversion with a resultant decrease in DC voltage level; and so-called boost-buck converters, which can produce an increased or a decreased voltage level in DC-DC-conversion.
Power factor is a concept that describes the effectiveness of a load at converting supplied volt-amperes to watts. It is generally defined as the ratio of real power (I2R or watts) to apparent power (I2Z or volt-amperes). For many types of loads, this ratio is less than unity. In particular, rectification circuitry in an electric power supply can cause the power supply to appear as a load that has a less-than-unity power factor. Because loads with low power factor are undesirable for a variety of reasons, and because some utilities and government agencies mandate that loads must exhibit prescribed minimum power factors under certain conditions, power factor correction techniques are normally employed. For switching power supplies, power factor correction is usually accomplished with an input DC-DC converter. An input DC-DC converter is a switching converter placed between the rectification circuitry and the bulk capacitors that feed the output converters. The switching element of the input DC-DC converter is controlled in an effort to shape the input current of the power supply so that it matches the voltage waveform of the AC mains, resulting in improved power factor for the power supply and reduced harmonics in the current waveform.